Microsoft revealed last week that 400 million active machines are now running Windows 10. The latest update on usage comes just three months after the software maker revealed in June that 350 million devices were running Windows 10. While Microsoft claims the adoption rate of Windows 10 is 115 percent faster than Windows 7, the company originally claimed it would have 1 billion devices running Windows 10 by 2018. Microsoft revised its timeline in July noting that "it will take longer than FY18 for us to reach our goal of 1 billion monthly active devices." Microsoft's latest stats show why it will take longer to hit 1 billion devices running Windows 10.

Before the free Windows 10 upgrade offer ended in July, nearly 30 million additional machines were running Windows 10 each month. That growth has stalled to around 16.6 million each month during the past three months, at a time when the upgrade offer expired and PC sales are typically slower. It is not clear how well growth will progress for the rest of the year, but Microsoft is still focused on reaching its goal of 1 billion Windows 10 devices.

That goal is clearly going to take a little longer than Microsoft initially expected, and the company will not likely hit its target until 2019 or 2020 unless there is a significant boost to sales of devices running Windows 10. Microsoft had been placing some of its hopes on Windows Phone to help push its Windows 10 target, but the company has realigned its phone efforts and Windows Phone has dipped below 1 percent market share as a result. Microsoft has not said when it expects to hit its 1 billion devices target for Windows 10.